Crude steel is produced by those methods roughly grouped into a blast furnace-converter process and an electric furnace process. Of such methods, the electric furnace process gives steel by melting a starting iron material such as scrap or reduced iron with heat derived from electric energy and, where desired, by further refining the melt. In the electric furnace method, the scrap is now a dominant feed stock. Recently, however, the reduced iron has been in growing demand so as to compensate for shortage of the scrap and also to meet with the need for steel products of high quality.
One process for the production of reduced iron is disclosed for instance in Japanese Unexamined Patent Publication No. 63-108188. This prior art process is comprised of charging an iron ore and a solid charging material in a horizontally rotary hearth furnace, of stacking the respective layers one on the other, of heating the resulting layer from above by means of radiant heat transfer, thereby reducing the iron ore, whereupon a reduced iron is obtained.
In general, in a rotary hearth furnace for use in ore reduction, a series of operations such as stack of feed stock on a hearth, preheating, reduction and discharge is effected while the hearth is rotated once. In order to improve productivity to as high an extent as possible in keeping pace with such series of operations, a feed port 6 for charging feed stock and a discharge port 7 for taking out the treated reduced ore disposed adjacent to each other as shown in FIG. 1. On a rotary hearth 3 from the feed port 6 to the discharge port 7, as viewed in FIG. 1, is stacked feed stock a layer (in FIG. 2) t composed of an iron ore and a solid reducing material as seen in FIG. 2 that is taken along a line A--A which appears in FIG. 1. The feed stock is covered by a furnace body 3 lined on its upper surface and side surfaces with a refractory material, and a burner 5 is located upstream of the furnace body. By use of fuel gas, heavy oil or the like, the burner 5 heats up the feed stock t stacked on the rotary hearth 4. With such burner used as a source of heat, the feed stock stacked on the rotary hearth 4 is heated up so that the ore a is reduced by a carbon material in the feed stock.
Here, the inner furnace temperature is usually maintained at around 1300.degree. C. Upon completion of the reduction treatment, the ore is converted to a reduced ore which, when taken as reduced out of the furnace, is vulnerable reoxidation owing to its high temperature and hence tends to suffer deteriorated quality of the finished steel product. The reduced ore of elevated temperature is also liable to render the discharge port 7 and other neighboring equipment and facilities susceptible to impairment or short service life. To cope with those defects, it has been considered thus far that a reduced ore could be cooled on the movable hearth with use of a cooler of an air, water or like type, followed by discharge and recovery of the cold steel product. This sort of countermeasure leaves the problem, however, that it calls for use of utilities such as gas, water and the like and moreover complicated equipment with added investment. Besides and disadvantageously, energy loss takes place unless good use is made of energy derived from heat exchange by gas, water or the like.